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Re: [TowerTalk] Dipole questions for the Liberal Arts Major

To: "Jim Smith" <>,"TowerTalk Reflector" <>
Subject: Re: [TowerTalk] Dipole questions for the Liberal Arts Major
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 31 Aug 2005 03:02:46 -0400
List-post: <>
> Would it make any sense to cut one of the bow tie wires to
a different
> frequency than the other to get a double tuned circuit
type of
> response?

I've never really investigated that so I have no idea if it
is better or not.

The main limitation in bandwidth is the stored energy in the
abrupt end boundary where the electric field concentrates.
There is a great deal of stored energy at that point (high
Q). I use a thin metal tubing spreader (as part of the
antenna) to space the ends out. By making the antenna "fat"
at the ends the transition is spread out and the electric
field is not concentrated.

That link to Cebik's page using a 1/4 wl transformer shows
another useful method but the dipole has to be at an
appropriate height to use that effect. If you have the
dipole at a height (or in surroundings) where the dipole's
feed impedance is lower than 75 ohms or so mixing of line
sections can hurt bandwidth. My dipoles are generally low
(under 1/4 wl high) for NVIS applications or at 5/8th wl
high for DX, so the 50/75 ohm sections don't work out well
for me.

Tim's suggestion of open wire and a tuner is a good one. I
keep thinking of doing that but my situation is such that
the tuner has to be remotely located at the antenna. Getting
whacked by lightning several times a year, I don't want to
bring an above ground feeder into the shack. Someday I'll
build a remote tuner.

I've retired all my kilowatt Johnson matchboxes. When I use
tuners I use ATR30's. They have just as good balance, match
a wider impedance range over wider frequency range, and
handle as much or more power without problem. When I used
the Johnson KW MBoxes I modified them so the extra coax
connector on the rear went to a different link tap.That way
I could extend the matching range. Less turns on the link
means wider matching range, but lower power rating and less
bandwidth (higher Q).

73 Tom


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